The present invention relates to a device and method for improving heating of food in a microwave oven. The invention addresses the problems of cold spot, uneven heating and splattering that traditionally occur when foods are heated in a microwave oven. In particular, the present invention proposes a method for handling and evenly heating frozen food that can be economically, conveniently and rapidly served in a foodservice location to the consumer. The invention also relates to a stand-like device adapted to receive a food container for improving the reheating of the food in a microwave oven.
Microwave reheating of frozen meals provides convenience for people seeking quick meal solutions because of the rapid thermal energy transfer into the food materials by the microwaves. However, microwave heating of frozen food is, in general, perceived as difficult and has many problems associated with it, such as overcooking of some portions of meals, cold spots, burnt edges and sauce splatter. In many instances, the meal experiences serious burnt spots yet some places are still very cold, even often below 0xc2x0 C. Furthermore, the concentration of the microwave energy on spots tends to create local boiling of the water contained in the food and/or sauce and therefore splattering off the dish.
Many attempts have been made in the past to solve this issue of re-heating frozen food in a more effective way.
It has previously been proposed in U.S. Pat. No. 4,351,997 to provide a modified form of tray structure to attempt to provide a more even heating of foodstuff in the tray when exposed to microwave radiation. This prior art has a bottom wall of microwave-transparent material and an upwardly-extending peripheral wall that is outwardly curved at its upper end to define a horizontal peripheral rim. The rim is partly or completely coated with a material that is reflective and opaque to microwave radiation, such as aluminum foil. The peripheral wall also is partly or completely coated with the foil material. The effect of this aluminum foil coating on the wall is to provide reflection of microwave energy toward the center of the tray.
U.S. Pat. No. 4,626,641 describes an embodiment in which a similar structure is provided. In addition to the provision of aluminum foil in the side wall of a tray, the foil also extends into the base of the container but leaves a rectangular open area in the bottom wall.
WO 92/19511 relates to a tray useful for the microwave cooking of prepared foodstuff that comprises an outer layer formed of paperboard or molded plastic to which is laminated an inner polymeric film layer. A layer of microwave-reflective material, usually aluminum foil, is positioned between the outer and inner layers in the location of the peripheral wall of the tray and in a pattern in a portion of the bottom wall.
JP 09-369450 relates to a container for a microwave oven that comprises a first microwave reflecting plate placed along a circumferential lateral parts and a second microwave reflecting plates in the base section of the container.
It is apparent from the prior art that attempts have essentially been made to provide energy transmission structures with reflective material placed in locations that enable energy transfer from the edges and corners of the plate to a more central area of the plate. However, experimental trials have shown that these structures are, by themselves, insufficient to overcome the problems of uneven heating. In particular, cold areas are still present in the food despite the presence of these structures. Thus, improvements in these devices are needed and are provided by the present invention.
The present invention aims at providing a satisfactory solution for evenly heating a frozen food in a plate by adopting a different approach where not only energy transfer is carried out but more importantly a modification of the overall heating pattern inside the food block is achieved by changing the wavelength inside the food. Thus, the present invention aims at providing a convenient and easy way for improving microwave reheating of food while enabling the use of standard dishes such as ceramic and ceramic-like plates that may commonly be found in restaurants, cafeterias, hotels, or other foodservice locations.
The present invention specifically relates to a method for ensuring a more uniform heating of frozen food by microwaves. This method is conducted by providing food in a portion having a predetermined size and shape; and providing a container adapted for receiving and reheating with microwaves the food portion. The container includes a supporting cavity having peripheral sides and a bottom side, with the peripheral sides of the container being circumferentially shielded by a microwave reflective material such that the microwave reflective material on the peripheral sides forms a circumference which has axial and transverse distances that are determined so as to change the wavelength of resonant modes in the supporting cavity. The method includes placing the portion of food in the supporting cavity; and heating the food and container with microwaves so as to expose the food to the changed wavelengths of resonant modes in the cavity thereby resulting in a more uniform heating food pattern and a more uniform heating of the food.
It has been surprisingly found that it is possible to render the heating of the food in the container relatively even by shortening the wavelength of the resonant modes inside the cavity of the food, especially for the transverse electric (TE) modes. Shortening of the wavelength and supporting of specific resonant modes can be obtained more specifically by determining the distances that separate the microwave reflective material when placed cicumferentially along the side of the container in a manner to promote modes that show a more even electric field distribution.
In a preferred embodiment, the microwave reflective material forms a resonating layer having a transverse dimension of 15 cm or less. Preferably, the transverse dimension of the resonating layer is of less than 13 cm, even preferably of from about 10 to 12 cm. Preferably, the axial dimension of the resonating layer is of 20 cm or less, even preferably of less than 18 cm.
Therefore, the resonating layer can be regarded as a resonator or waveguide where shortened standing waves occur inside the food. As the transverse dimension of the resonating layer is shortened, transverse electric (xe2x80x9cTExe2x80x9d) modes that carry more power than transverse magnetic (xe2x80x9cTMxe2x80x9d) modes are propagated in the cavity of the food. In particular, the TE modes that are permitted for the selected geometry are those with a more concentrated heating in the center area of the cavity. More preferably, TE modes that are predominantly supported in the food cavity are TEn,m,1 modes wherein n may be 0, 1 or 2, m may be 0 or 1, and 1 may be 0 or 1.
Preferably, the microwave reflective material forms a resonating layer having a closed rounded contour. Even more preferably, the resonating layer has an oval contour with an axial dimension of less than 20 mm, preferably less than 18 mm. In an alternative embodiment, the resonating layer has a circular contour. In a preferred embodiment, the bottom side is substantially free of shielding material.
Remarkable results have been obtained by using such shapes and dimensions of a resonating layer. Comparative trials have shown that these are the preferred configurations for evenly heating food in the container.
The invention also relates to a method of using a container having the resonating layer as previously defined wherein, prior to placing the food portion in the container, the food portion is removed frozen from a package of defined shape and size which are predetermined to substantially match the shape and size of the cavity of the container. Therefore, the frozen food can be produced and packaged in the manufacturing facility according to specific standards of size and shape that are predetermined to provide optimal re-heating properties afterwards. Therefore, the food product may be packaged in a package that is not the final heating tray with its resonating layer so as to reduce the packaging costs. Therefore, contrary to the package which may usually be disposable, the final heating tray may be a dish made of a non-disposable material for repeated uses in a foodservice location such as a restaurant, a fast-food or a cafeteria.
The invention also relates to a stand-like device for improving the heating of food in a container. This device includes a support body of a shape and size adapted for receiving the container in a removable manner and a circumferential shielding surface of microwave reflective material adapted to be positioned adjacent to peripheral sides of the container.
In a preferred embodiment, the microwave reflective material of the circumferential shielding surface is defined by axial and transverse distances that are determined so as to change the wavelength of resonant modes inside the food thereby resulting in a more uniform heating food pattern. Still in a preferred embodiment, the support body has a wedge-shaped receiving surface to snuggly engage the complementary surface of the container and leverage means pivotally associated to the body to act on the bottom surface of the container to disengage the container from the wedge-shaped receiving surface.
The benefit of such a supporting stand is that it allows to more evenly heat the food in a microwave oven while reusable standard dishes such as traditional dishes can be used. Therefore, reusable standard dishes can become more effective for microwave reheating of food as compared to the use of the same dishes without the supporting stand. The supporting stand may be of a convenient use in kitchens of restaurants, hotel chains, transportation or other foodservice areas where paper or plastic dishes can not decently be used but only food in traditional ceramic or ceramic-like dishes can be served to the customer.